1. Field of the Invention
This invention relates to a gas-detection sensor and more particularly to a solid state compact ion gauge which is micro-machined on a semiconductor substrate.
2. Background Information
Various devices are currently available for determining the quantity and type of molecules present in a gas sample. One such device is the mass-spectrometer.
Mass-spectrometers determine the quantity and type of molecules present in a gas sample by measuring the mass-to-charge ratio and quantity of ions formed from the gas through an ionization method. This is accomplished by ionizing a small sample and then using electric and/or magnetic fields to find a charge-to-mass ratio of the ion. Current mass-spectrometers are bulky, bench-top sized instruments. These mass-spectrometers are heavy (100 pounds) and expensive. Their big advantage is that they can be used to sense any chemical species.
Another device used to determine the quantity and type of molecules present in a gas sample is a chemical sensor. These can be purchased for a low cost, but these sensors must be calibrated to work in a specific environment and are sensitive to a limited number of chemicals. Therefore, multiple sensors are needed in complex environments.
One of the methods utilized to determine the nature of a molecular species is to determine its molecular weight. This is not a unique property of a molecule, since the same set of atoms that constitute a molecule can be bonded together in a variety of ways to form molecules with differing toxicities, boiling points, or other properties. Therefore, in order to uniquely identify a particular molecular compound, the structure must be identified. A well-established technique for determining the molecular structure of molecules is the dissociative ionization of molecules and then determining the quantity and mass to charge ratio of the resulting ion fragments, also known as the cracking pattern. The general technique is referred to as mass spectroscopy.
To determine the mass to charge ratio of an ion, a variety of methods are utilized which causes a separation of the ions either by arrival at a detector over a period of time, or by causing a physical displacement of the ions. The number of detectors simultaneously used determines the speed and sensitivity of the device. Techniques that scan the ion beam over a single detector are referred to as mass-spectrometers and those that utilize multiple detectors simultaneously are referred to as mass-spectrographs. Mass-spectrographs can also be scanned by utilizing an array that covers a subset of the full range of mass to charge ratios; scanning multiple subsets allows coverage of the entire mass range. In order to provide a micro-miniature mass-spectrograph, there is a need for a micro-miniature mass separator that can be used in that micro-miniature mass-spectrograph.
Typically, a solid state mass spectrograph can be implemented on a semiconductor substrate. FIG. 1 illustrates a functional diagram of such a mass spectrograph 1. This mass spectrograph 1 is capable of simultaneously detecting a plurality of constituents in a sample gas. This sample gas enters the spectrograph 1 through dust filter 3 that keeps particulate from clogging the gas sampling path. This sample gas then moves through a sample orifice 5 to a gas ionizer 7 where it is ionized by electron bombardment, energetic particles from nuclear decays, or in a radio frequency induced plasma. Ion optics 9 accelerate and focus the ions through a mass-filter 11. The mass-filter 11 applies a strong electromagnetic field to the ion beam. Mass-filters that utilize primarily magnetic fields appear to be best suited for the miniature mass-spectrograph since the required magnetic field of about 1 Tesla (10,000 gauss) is easily achieved in a compact, permanent magnet design. Ions of the sample gas that are accelerated to the same energy will describe circular paths when exposed in the mass-filter 11 to a homogenous magnetic field perpendicular to the ion's direction of travel. The radius of the arc of the path is dependent upon the ion's mass-to-charge ratio. The mass-filter 11 is preferably a Wien filter in which crossed electrostatic and magnetic fields produce a constant velocity-filtered ion beam 13 in which the ions are disbursed according to their mass/charge ratio in a dispersion plane that is in the plane of FIG. 1.
A vacuum pump 15 creates a vacuum in the mass-filter 11 to provide a collision-free environment for the ions. This vacuum is needed in order to prevent error in the ion's trajectories due to these collisions.
The mass-filtered ion beam is collected in an ion detector 17. Preferably, the ion detector 17 is a linear array of detector elements that makes possible the simultaneous detection of a plurality of the constituents of the sample gas. A microprocessor 19 analyses the detector output to determine the chemical makeup of the sampled gas using well-known algorithms that relate the velocity of the ions and their mass. The results of the analysis generated by the microprocessor 19 are provided to an output device 21 which can comprise an alarm, a local display, a transmitter and/or data storage. The display can take the form shown at 21 in FIG. 1 in which the constituents of the sample gas are identified by the lines measured in atomic mass units (AMU).
Preferably, mass spectrograph 1 is implemented in a semiconductor chip 23 as illustrated in FIG. 2. In the preferred spectrograph 1, chip 23 is about 20 mm long, 10 mm wide and 0.8 mm thick. Chip 23 comprises a substrate of semiconductor material formed in two halves 25a and 25b that are joined along longitudinally extending parting surfaces 27a and 27b. The two substrate halves 25a and 25b form at their parting surfaces 27a and 27b an elongated cavity 29. This cavity 29 has an inlet section 31, a gas ionizing section 33, a mass filter section 35, and a detector section 37. A number of partitions 39 formed in the substrate extend across the cavity 29 forming chambers 41. These chambers 41 are interconnected by aligned apertures 43 in the partitions 39 in the half 25a that define the path of the gas through the cavity 29. Vacuum pump 15 is connected to each of the chambers 41 through lateral passages 45 formed in the confronting surfaces 27a and 27b. This arrangement provides differential pumping of the chambers 41 and makes it possible to achieve the pressures required in the mass filter and detector sections with a miniature vacuum pump.
The inlet section 31 of the cavity 29 is provided with a dust filter 47 that can be made of porous silicon or sintered metal. The inlet section 31 includes several of the aperture partitions 39 and, therefore, several chambers 41.
FIG. 3 shows the detector array 17 having MOS capacitors 67 which are read by a MOS switch array 69 or a charge coupled device 69. The detector array 17 is connected to an array of Faraday cups formed from a pair of Faraday cup electrodes 71 which collect the ion charge 73.
A cross-section of the all-silicon mass spectrograph 1 is shown in FIG. 4. The top 25a and bottom 25b silicon pieces are preferably bonded by indium bumps and/or epoxy, which are not shown. The first step in the fabrication of the all-silicon mass spectrograph 1 is the etching of alignment marks in the silicon substrate 25. This assures proper alignment of the etched geometries with the cubic structure of the silicon substrate 25. Once the alignment marks are etched, 40 μm deep chambers 41 are etched in each half 25a and 25b of the silicon substrate 25. These chambers are etched using an anisotropic etchant such as a potassium hydroxide etching agent or ethylene diamine pyrocatechol (EDP). After the chambers are formed, the orifices between the chambers are formed by etching 10 μm deep features. These orifices are also etched using the anisotropic etching agent.
The miniaturization of mass spectrograph 1 creates various difficulties in the manufacture of such a device.
In any ionic mass spectrometer or charge sensing device, there must be some means to collect the charge and determine its magnitude. For high performance devices, sensitivity of 10's of charges at speeds of 10's of kilocycles is required. An additional resolution constraint is mandated for mass spectrographs: the detector pitch must be smaller than the ion beam while insuring that the ion beam is not missed due to inter detector spacing of non-contiguous detector elements. As detector pitch is reduced, smaller displacements (i.e., better mass resolution in a miniaturized package) can more readily be discerned.
In the present state of the art, charge multiplication devices and high gain current sensors have been utilized. Charge multiplication devices require high voltages (>1000 volts) in order to operate. This is difficult to implement on a silicon chip where voltages are generally less than 100 volts. High gain current amplifiers, often referred to as electrometers, operate at low voltages and can be used to measure total charge. Electrometers typically found in laboratory instruments are useful for currents on the order of 1×10−14 amperes. However, this sensitivity is at the expense of speed, with response time approaching several seconds for these low current values.
Another charge sensor that is typically used for the detection of light and high energy particles is a charge-coupled device (CCD). Photoelectrons generated at a capacitor or charge injection from a high energy particle onto a capacitor are moved by the CCD to a charge sensitive amplifier and converted to a voltage signal which can be sensed. CCDs are capable of sensing low amounts of charge (some as low as 10's of charges per read cycle) with read rates in the 10's of kilocycles, but require a passivating dielectric over the charge storage capacitor to protect the active CCD semiconductor layers from environmental degradation. This dielectric precludes sensing of low energy molecular and atomic ions.
High speed and low charge sensing devices capable of accurately detecting low energy molecular and atomic ions are required to effectively miniaturize ionic gas sensors. Accordingly, there is a need for a solid-state detection for sensing low energy charge particles.
If the reader desires further background information, reference can be made to the following:                A User's Guide to Vacuum Technology, 2nd Edition, by John F. O'Hanlon (1989, John Wiley & Sons), Chapter 5, pp. 75–99;        Building Scientific Apparatus—A Practical Guide to Design and Construction, 2nd Edition, by John H. Moore et al., (1989, Addison-Wesley Publishing Company, Inc.), pp. 80–83;        Micromachined Devices and Components, Proc SPIE, Vol. 3514, p. 431, “Comparison of Bulk- and Surface-Micromachined Pressure Sensors,” William P. Eaton et al.        U.S. Pat. No. 5,386,115 to Freidhoff et al., entitled “Solid State Micro-machined Mass Spectrograph Universal Gas Detection Sensor”;        U.S. Pat. No. 5,492,867 to Kotvas et al., entitled “Method for Manufacturing a Miniaturized Solid State Mass Spectrograph”;        U.S. Pat. No. 5,530,244 to Sriram et al., entitled “Solid State Detector for Sensing Low Energy Charged Particles”; and        U.S. Pat. No. 5,536,939 to Freidhoff et al., entitled “Miniaturized Mass Filter.”        
Each of the noted patents is assigned to the present Assignee and is incorporated herein by reference.
While these patents describe a mass filter that has served its intended purpose, there is still a need to eliminate the mass filter so that a low cost and compact ion gauge can be used in high vacuums and ultra-high vacuums. The use of silicon micromachining and devices allows for a low cost and compact ion gauge. Such a compact ion gauge would provide new capabilities in vacuum process equipment by placing a network of pressure sensors on vacuum tools rather that a single one. With the sensors being networked on a process tool, leak checking and process variability can be reduced which will increase efficiency and process yield.